Luteolysis in sheep is associated with uterine secretion of pulses of prostaglandin F2 alpha (PGF2 alpha) due to the action of luteal oxytocin on endometrial oxytocin receptors. For pregnancy to become established inhibition of oxytocin receptors is important as an antiluteolytic mechanism. The maternal recognition of pregnancy in cattle and sheep involves production, by the trophoblast, of a type 1 interferon (IFN-tau) that suppresses uterine development of oxytocin receptors and the generation of luteolytic episodes of PGF2 alpha. The action of IFN-tau in surgically prepared unilaterally pregnant ewes was investigated. Finn-Dorset ewes were anaesthetized on day 6 or 7 of the oestrous cycle and one uterine horn was surgically isolated at the uterine bifurcation from the body of the uterus. Ewes were mated at the subsequent oestrus either by a fertile or by a vasectomized ram and killed on day 13 or 16 after mating. On day 16, in the non-pregnant ewes, there was no measurable uterine IFN-tau but there were high concentrations of oxytocin receptors in both horns. In the pregnant ewes on day 16 after mating, the oxytocin receptor concentration was 45 +/- 11 fmol mg-1 protein in the pregnant horn and 585 +/- 131 fmol mg-1 in the non-pregnant horn. Antiviral activity was 5.8 x 10(7) +/- 5.2 x 10(7) U ml-1 in the pregnant horn and 2.9 x 10(3) +/- 1.2 x 10(3) U ml-1 in the non-pregnant horn. Thus, 16 days after mating, the pregnant horn exhibited high antiviral activity but oxytocin receptors were suppressed, while in the same endocrine environment (characteristic of pregnancy) there were low IFN-tau and high oxytocin receptor concentrations in the isolated horn equivalent to those expected at the onset of luteolysis. In situ hybridization to ovine mRNA encoding the oxytocin receptor and autoradiographic studies using the 125I-labelled oxytocin antagonist d(CH2)5[Tyr(Me)2,Thr4,Tyr-NH2(9)]-vasotocin both showed that the large amount of oxytocin receptor message and binding sites in the endometrium of the isolated horn were localized in the luminal epithelium. Immunocytochemical studies showed that there was a suppression of oestradiol receptors in the pregnant horn but high concentrations equivalent to those at oestrus were present in the isolated horn. The content of progesterone receptors was low in the stromal tissue only in both horns, a pattern of localization similar to that seen in the late luteal phase and in early pregnancy.(ABSTRACT TRUNCATED AT 400 WORDS)